Porous honeycomb structures have been widely used as filters for collecting and removing granular material contained in dust-bearing fluids like diesel engine exhaust gas or as catalyst supporters for supporting catalyst components for purifying exhaust gases. Also, it is well known that refractory particles such as a silicon carbide (SiC) particle are used as composition material for such a honeycomb structure.
As a specifically relating art, for example, JP-A-6-182228 discloses a porous silicon carbide catalyst carrier of a honeycomb structure obtained by a preparation method that involves forming in a desired shape a silicon carbide powder having a given specific surface area as well as containing impurities, used as the starting material, drying and subsequently firing in a temperature range of 1600 to 2200° C.
On the other hand, JP-A-61-26550 discloses a method of manufacturing a refractory bearing vitrifying material that features adding vitrifying material to a readily oxidized material or a refractory composition containing a readily oxidized material, mixing the resultant material with a binding agent, kneading and molding, and subsequently bare burning a molded body thus molded in a furnace under a non-oxidizing atmosphere. JP-A-8-165171 discloses a silicon carbide molded article molded subsequently to the addition of an organic binder and an inorganic binder of a clay mineral base, glass base, or lithium silicate base to a silicon carbide powder.
In addition, JP-A-6-182228 above presents as a conventional method for manufacturing a porous silicon carbide sintered compact a manufacturing method that includes adding a binding material such as vitrified flux or clay to carbonate particles to be an aggregate, molding, and subsequently sintering and hardening the resulting molded body at the temperature at which the aforementioned binding material.
Furthermore, JP-B-61-13845 and JP-B-61-13846 disclose a refractory particle graded to a specified particle size, comprising silica sand, ceramic pulverized substances, metal oxides such as Al2O3, TiO2, ZrO2, silicon carbide, nitrides, borides, or other refractory materials, and a suitable average particle size of a refractory particle, a particle size distribution of a refractory particle, the porosity of a tubular body, the average pore diameter of a tubular body, the pore volume of a tubular body, and the partition wall thickness of a tubular body, and the like, with respect to a high temperature ceramic filter formed in a porous, bottomed tubular shape with a refractory bonding material such as liquid glass, flit, glaze, and the like.
Additionally, JP-B-8-13706 discloses a silicon carbide/metallic silicon composite having a structure integratedly joined via metallic silicon and a process for manufacturing the aforementioned composite using silicon carbide and metallic silicon formed by heat treating silicon accumulated biomass under a argon or nitrogen atmosphere.
With a burned form (necking) by means of the recrystallization of silicon carbide powder itself, disclosed in JP-A-6-182228 as described above, the silicon carbide component is evaporated from the surfaces of the silicon carbide particles, and the evaporated component is condensed in the contact portions (neck portions) between particles, and thus the neck portions grow to yield the bonded state. However, evaporation of the silicon carbide requires a very high firing temperature, which leads to a high cost. Also, a material of a high thermal expansion coefficient needs to be fired at a high temperature. These create the problem of decreasing the firing process yield.
In addition, production of a filter of a high porosity, particularly with a porosity of 50% or more, by firing via recrystallization of the aforementioned silicon carbide itself, leads to inhibition of the growth of the neck portion due to insufficient performance of the sintering mechanism, thereby creating the problem of decreasing the strength of the filter.
Furthermore, the aforementioned material, with a very high thermal conductivity of 30 W/mK or more, has an advantage in suppressing a local heat evolution. However, for example, when a filter is used that has a system of oxidizing and combusting a particulate with a catalyst carried to continuously perform reproduction, it requires much time to raise the temperature of the carrier because of a small amount of particulate accumulated and an easy emission of heat. Therefore, it needs time to increase the temperature to a temperature at which the catalyst functions, which also creates the problem of producing cinders of the particulate resulting in a decrease in reproduction efficiency.
An approach to bonding a raw material of a silicon carbide via glass material, disclosed in JP-A-61-26550 and JP-A-6-182228, can handle the matter using a low firing temperature of 1,000 to 1,400° C. However, for example, use of a sintered body prepared by this method as a material for a diesel particulate filter (DPF) for removing a particulate contained in exhaust gases discharged from a diesel engine, causes the problem of locally generated heat due to a small thermal conductivity when combusting particulates collected and accumulated on the filter.
Moreover, while the filters disclosed in JP-B-61-13845 and JP-B-61-13846 are porous, they are a bottomed cylinder with a thick partition wall of 5 to 20 mm and thus cannot apply to conditions of a high space velocity (SV) like in a filter for purifying automobile exhaust gas.
In addition, according to a composite and the manufacturing method thereof indicated in JP-B-8-13706, the composite can be made to be porous; however, it is not easy to ensure a sufficient porosity when the composite is utilized as a filter, and particularly it is difficult to use the composite as a filter for collecting and removing granular materials contained in a dust-bearing fluid like diesel engine exhaust gas.
The present invention has been made taking into consideration these conventional circumstances, and the object of the invention is to provide a honeycomb structure which can be costlessly produced at a relatively low firingburning temperature while containing refractory particles such as a silicon carbide particle and which is sufficiently porous, have a high specific surface area and can be suitably used as a filter for purifying automobile exhaust gas under high SV conditions by means of treatment of clogging, catalyst supporting, etc. and the manufacturing method thereof.